The invention relates to a method and a circuit arrangement for transmitting signals from an output of a first circuit to an input of a second circuit, where the circuits have a first and a second operating voltage and a first and a second ground potential applied to them, and where variable potential differences can arise between the ground potentials.
When signals are transmitted between two circuits having a potential offset with respect to one another, the transmitted signals may become distorted. If, as an example, a control unit in a motor vehicle is connected to a different ground point than the associated power output stage, considerable potential differences can arise, with the result that signals transmitted from the control unit to the power output stage may be distorted. The potential offset may even be a multiple of the operating voltage, with the result that even digital signals can no longer be reliably evaluated.
The object of the present invention is therefore to provide a method and a circuit arrangement for transmitting signals, in which variable potential differences which arise between the circuits do not cause distortion.
This object is achieved for the method according to the invention in that a current controlled by the signal which is to be transmitted flows from the output of the first circuit to a circuit point having a further potential applied to it and controls a further current, which emanates from the circuit point and is supplied to the input of the second circuit, and in that the further potential is chosen such that a respective voltage enabling the current and the further current is present between the output of the first circuit and the further potential, on the one hand, and between the further potential and the input of the second circuit, on the other hand, for the potential differences which arise.
In the case of the circuit arrangement according to the invention, the output of the first circuit is provided with a controllable current source whose current flows in the direction of a circuit point having a further potential applied to it and controls a further current source, which is connected to the circuit point and whose current flows in the direction of the input of the further circuit, and the further potential is chosen such that a respective voltage enabling the current and the further current is present between the output of the first circuit and the further potential, on the one hand, and between the further potential and the input of the second circuit, on the other hand, for the potential differences which arise. The potential difference which can be overcome by means of the invention is limited only by the permissible collector/emitter voltage of the transistors which at least partly form the current sources.
According to one refinement of the circuit arrangement according to the invention, a sufficient flow of current through the current sources is ensured in that the further potential is lower than the first operating voltage by at least the sum of the emitter/collector voltage of a transistor which at least partly forms the current source and whose emitter has the first operating voltage applied to it, and the base/emitter voltage of a transistor which at least partly forms the further current source, and is lower than the second operating voltage by at least the sum of the collector/emitter voltage of the transistor which forms the further current source, and the base/emitter voltage of a transistor connected to the input of the second circuit.
When transistors of a different conductivity type are used, this is achieved in that the further potential is higher than the first ground potential by at least the sum of the emitter/collector voltage of a transistor which at least partly forms the current source and whose emitter has the first ground potential applied to it, and the base/emitter voltage of a transistor which at least partly forms the further current source, and is higher than the second ground potential by at least the sum of. the collector/emitter voltage of the transistor which forms the further current source, and the base/emitter voltage of a transistor connected to the input of the second circuit.
Advantageous developments of the circuit arrangement according to the invention focus on details for the further potential in the case of respectively positive and negative operating voltages for the circuits and the use of pnp or npn transistors for the current sources.
One development of the circuit arrangement according to the invention provides that, to produce the further potential, a circuit point carrying the further potential has the ground potentials or the operating voltages applied to it via a respective diode.
If the circuit arrangement according to the invention is used in a motor vehicle, the operating voltage of the second circuit, which contains power output stages, may fail briefly. So that the power output stages can be actuated again as quickly as possible when the operating voltage returns, another development of the circuit arrangement according to the invention provides that, to produce the further potential, a circuit point carrying the further potential is connected to the operating voltage of one of the circuits via a capacitor and a first diode, and to the ground potential of the other circuit via a second diode.
A preferred application for the circuit arrangement according to the invention involves the first circuit being a control circuit and the second circuit being a power stage in an electronic motor vehicle device.